Method for preheating scrap

ABSTRACT

The apparatus comprises a container for a scrap charge which is a clam shell type refractory-lined charging bucket, a movable refractory-lined burner hood equipped with a plurality of downwardly directed burners, said burner hood adapted to fit atop the container, a self-propelled transfer car upon which the container rests and an exhaust system. Low density or high density scrap charges in the container are transported to a preheater station. The refractory-lined burner hood is placed atop the container and the exhaust duct is connected to the bottom of the transfer car. Hot combustion gases formed in the burner hood flow downwardly through the scrap charges to heat the scrap. Spent hot combustion gases leave the container through the clam shell of the container to the exhaust system and are discharged to the atmosphere. Firing rates are regulated to preheat the low or high density scrap charges in an effective manner.

United. States Patent 91 Reinhold et a1. 2

in] 3,802,830 [451 Apr. 9, 1974 METHOD FOR PREHEATING SCRAP [75] Inventors: Richard J. Reinhold, Allentown;

Eugene M. Rudzki, Bethlehem; Bradford K. Pease, Allentown, all of Pa.

Bethlehem Steel Corporation, Bethlehem, Pa.

221 Filed: Mar. 28, 1972 2 1 Appl. No.: 238,870

[73} Assignee:

[52] US. Cl 432/9, 432/14, 432/19 [51] Int. Cl. F27b 14/14 [581 Field of Search 432/7, 9, l4, l9; 266/5 266/335; 75/43 [56] References Cited UNITED STATES PATENTS 3.589.692 6/1971 L'akin 432/9 3,307,935

3/1967 Lindholm 266/5 R Primary Examiner-lohn J. Camby [57] ABSTRACT The apparatus comprises a container for a scrap charge which is a clam shell type refractory-lined charging bucket, a movable refractory-lined burner hood equipped with a plurality of downwardly directed burners, said burner hood adapted to fit atop the container, a self-propelled transfer car upon which the container rests and an exhaust system.

Low density or high density scrap charges in the container are transported to a preheater station. The

refractory-lined burner hood is placed atop the container and the exhaust duct is connected to the bottom of the transfer car. Hot combustion gases formed in the burner hood flow downwardly through the scrap charges to heat the scrap. Spent hot combustion gases leave the container through the clam shell of the containerto the exhaust system and are discharged to the atmosphere. Firing rates are regulated to preheat the low or high density scrap charges in an effective manner.

10 Claims, 3 Drawing Figures PATENFEWR 1914 3302.830

SHEET 1 OF 2 METHOD FOR PREHEATING SCRAP BACKGROUND OF THE INVENTION Steelmaking processes, for example, open-hearth, basic oxygen, electric arc and the like, use cold steel and/or iron scrap as at least a portion of the raw material charges used in steelmaking furnaces. Generally about 30 percent to about 60 percent of the raw material charge in the open-hearth process is scrap, although some shops may charge 100 percent cold scrap. The basic oxygen process uses about 25 percent to 30 percent scrap in the raw material charge. The electric arc process uses substantially 100 percent scrap. Scrap is included in the raw material charge because it is readily available and is generally cheaper than hot metal. Then, too, some plants may not have a sufficient source of hot metal to make up the required raw material charge. The use of scrap'also conserves the available iron ores and concentrates for future use.

Steel scrap is broadly classified as light scrap or heavy scrap. Light scrap includes automobile shredded sheet, sheet trimmings, plate trimmings, stamping and pressing trimmings, flashing from forgings, turnings from machining operations, sheet and strip which is unsuitable for use, and the like. Light scrap is frequently contaminated with oil and water introduced during processing. Heavy scrap includes ingot crops and ends, crop ends of blooms and billets, structural steel ends, rail ends, unusable plate, bars, piles and the like and is generally not contaminated with oily matter. iron can also be used as a portion of the scrap charge. Iron can be in the form of broken molds, obsolete heavy machinery and the like. With varying proportions of light and heavy scrap, as described above, low density scrap charges or high density scrap. charges can be charged into the furnace.

Prior practices have been to charge cold scrap into the furnace used to produce steel. As a result, heat time was excessive because the scrap had to be heated in the furnace to melting temperature before refining took place in the steel-making process. Several attempts to preheat the scrap prior to charging into a steelmaking furnace have been made. The scrap has been heated by hot combustion gases passing upwardly or downwardly through the scrap. Waste hot combustion gases from the steelmaking processes have been used as have hot combustion gases formed by providing burners in a burner hood directly above or below the scrap in a charging bucket. The recent processes are exemplified by U. S. Pat. No. 3,514,279 issued May 26, 1970 to R. F. Jensen et al. entitled Preheating Scrap with Controlled Oxidation. Although excessive oxidation can be controlled, the problems of uniform heating of the scrap with no channelling of the hot gases, and the vaporization and burning of oils contaminating the scrap and subsequent overheating, resulting in melting and fusion of the scrap have not been solved.

A low densityscrap charge consisting primarily of light scrap is susceptible to severe oxidation and melting because of flame impingement on the scrap which is near the burners. Light pieces of scrap have a high surface area to weight ratio resulting in high thermal efficiency in the process of heating scrap. Oil which contaminates light scrap is easily ignited and once ignited burns with a high intensity flame causing the scrap to melt and fuse. The melted scrap passes downwardly through the solid scrap charge and either solidifies on the colder scrap near the bottom of the container, resulting in a large mass of scrap which cannot be removed from the container, or the melted scrap solidifies in the clam shell thereby freezing the clam shell in the closed position. The fusion of the scrap can also close voids in the scrap, thereby causing non-uniform heating of the scrap. The vaporization of the oils causes dense, heavy, noxious fumes which can be emitted from the container.

A high density scrap charge consisting primarily of heavy scrap requires excessive heat input to raise the temperature of the scrap to the desired temperature because the heavy pieces of scrap have a low surface area to weight ratio resulting in poor heat transfer to the scrap and poor thermal efficiency. Then, too, heavy scrap is bulky and the desired volume of scrap which can be charged into a container is often decreased because weight limitations of either the container or the lifting equipment are reached before the container is full. Then, too, large voids can be formed in the scrap charge resulting in severe channelling and nonuniformity of heating of the scrap.

Because the use of burners directly'above the scrap charge can cause impingement of the flame on the scrap, resulting in severe oxidation of the scrap, it has been suggested to use hot waste gases from the steelmaking processes to preheat scrap charges. The recycling of waste hot combustion gases from the steelmaking processes may be inefficient because the gases may not contain sufficient heat to significantly contribute to raising the temperature of the scrap.

It is therefore an object of this invention to provide an apparatus and an effective method to preheat scrap which does not result in excessive oxidation of the scrap, minimizes melting and fusion of the scrap, and does not cause ignition of oil contaminated scrap.

It is an object of this invention to provide an improved apparatus for preheating lowdensity and high density scrap charges wherein hot combustion gases are formed in a novel burner hood and flow downwardly uniformly from the burner hood to the mass of charged scrap in a-container and pass through the scrap so as to effect a uniform preheating of said scrap at an optimum rate of heatinput to a minimum amount of time.

Broadly, the apparatus of the invention includes a container having the interior lined with refractory materials, a refractory-lined burner hood resting atop the container and equipped with a pluralityof downwardly directed burners, a self-propelled transfer car upon which the container is supported, said transfer car having a downwardly converging annular passage extending through the floor of the car and an exhaust system beneath the floor of the shop and connected to the annular passage in the transfer car by means of a slidable duct whereby the spent hot combustion gases and vaporized oil are exhausted.

The method of the invention includes charging light and/or heavy scrap into a container supported on a selfpropelled transfer car, transporting the container to a preheater. station, placing the refractory-lined burner hood atop the container, heating the low or high density scrap charge by hot combustion gases passing downwardly through the scrap charge for a time and at a rate to raise the temperature of the scrap to a mean temperature and exhausting the spent hot combustion gases.

DESCRIPTION OF THE DRAWINGS FIG. 1 is an elevational view, partly in section, of the I apparatus of this invention.

FIG. 2 is a cross-sectional elevational view of the apparatus of this invention.

FIG. 3 is a view taken along line 3-3 of FIG. 1.

DESCRIPTION OF A PREFERRED EMBODIMENT Referring to the drawings, FIG. 1 shows a preheater station comprising vertical structural steel beams l 1 and 12 mounted on the floor 13 of an electrical arc furnace shop and a horizontal structural steel beam 14.

supported by the beams 11 and 12. A pair of chains 15 and 15a and hoist mechanism (not shown) are mounted on the horizontal beam 14. I

A portable container 16, which is a clam shell type refractory-lined charging bucket, containing a load of scrap 16b is shown in position in the preheater section 10. The portable container 16 is supported on a selfpropelled movable transfer car 17. The transfer car 17 comprises a floor portion 17a supported on a box-like structural steel framework 17b and two downwardly depending structural steel members 170. The transfer car is mounted on four double flanged wheels 18, two of which are shown, by means of axles (not shown) attached tothe framework 17b and structural members 17c. The flanged wheels 18 of the transfer car 17 are v movably supported on tracks 19 which extend from a swingable partible jaws 23 and 23a. Each of the parti- I ble jaws 23 and 23a is supported by two opposed structural members 24 spaced 180 (only one of which is shown for each jaw) pivotally fastened to opposed structural steel brackets 25, also spaced 180 (one of which is shown). The brackets 25 are fastened. as by welding, to the outer surface of the vertical steel shell 16a. Each of the partible jaws 23 and 23a has a bracket 26 and 26a at the periphery thereof which is provided with an opening 27 and 27a whereby appropriate equipment, for example, overhead cranes and attendant crane hooks (not shown) can be attached to open the partible jaws 23 and 23a. Each of the partible jaws 23 and 23a is provided with a semi-cylindrical steel support member 23b fastened, for example, by welding, to the bottom thereof. The semi-cylindrical steel supports 23b define a cylindrical passageway between the partible jaws 23 and 23a and the annular passage 20 in the floor 17a of the transfer car 17. Steel support members 23b also function to support the container 16 on the transfer car 17. It will be noted that the diameter of the cylindrical passage defined by the steel supports 23b may be the same as, but is never less than, the diameter of the upper end of the annular passage 20.

Container 16 is provided with lifting means comprising a pair of opposed trunnions 21 and 21a and attendant structural steel -supports 22 and 22a whereby the container 16 can be lifted, for example, by a crane and carried to a steelmaking furnace for discharge of preheated scrap into the furnace.

In FIG. 1 a refractory-lined burner hood 28 supported by chains 15 and 15a is shown in position to be lowered atop the container 16. The burner hood 28 is lowered or raised by the chains 15 and 15a and hoist mechanism (not shown) attached to horizontal beam 14. The burner hood 28 is provided with a plurality of burners 29. Each burner 29 is provided with a fuel line 30 extending from manifold 30a to the burners 29 and an air line 31 extending from bustle 31a. The air manifold 31a is connected to an air intake pipe 32 extending downwardly through the floor 13 of the shop to a fan 33. Cooling-water is supplied to the base 540 of the burner hood 28 through cooling-water inlet hose 54c and its attendant pipe (not shown). The cooling water flows out of the base 540 through pipe 5411 and cooling water outlet hose 54f.

An exhaust system 34 comprising ductwork 35, an exhaust fan 36, a damping valve 37 and a slidable duct 38 is provided beneath the floor 13 of the shop. An opening 39 is provided in the floor 13 to allow the slidable duct 38 to be moved upwardly to contact the lower end of passageway 20 and to form a continuous passage therewith. I

FIG. 2 is a cross-sectional view in elevation of the container 16 supported on the floor 17a of the transfer car. .17 with the refractory-lined firing hood 28 in place atop the container 16. As noted previously, the container 16 is a clam shell type refractory-lined charging bucket comprising a vertical generally cylindrical steel shell 16a having a plurality of courses of refractory material laid-up against the inner surface thereof, to form a refractory lining shown generally at 41. The refractory lining 41 is supported by a refractory support which is a steel ring 45 welded to the steel shellv 16a. A second steel ring 42 having a generally inverted U- shape is attached to the top of the steel shell 16a to provide a substantially flat surface 43 whereupon the burner hood 28 can be supported.

Referring to FIG. 2, the partible jaws 23 and 230, which are not lined with refractory material because they are not exposed to high temperatures, for example over 750 F., are provided with a plurality. of orifices 52 and baffle plates 53 associated with the orifices 52. The orifices 52 allow the passage 20 in the transfer car 17 to the exhaust system shown generally at 34 in FIG. 1. The slidable duct 38 is shown in contact with the bottom of the floor of the transfer car 17 to thereby provide an essentially closed passage for the spent hot combustion gases to the exhaust systm shown in FIG. 1. While we have shown orifices 52 and baffle plates 53 in the partible jaws 23 and 23a to allow passage of spent hot combustion gases out of the container 16, it is not necessary to the method of the invention to use orifices 52 and battle plates 53 in the partible jaws 23 and 23:: since the hot combustion gases can be allowed to pass downwardly to the exhaust system by holding.

the partible jaws 23 and 23a partly open.

The refractory-lined burner hood 28 is shown in position atop the supporting ring 42 of the container 16. The burner hood 28 comprises a steel shell 54 which has the shape of a frustrum of a cone having a broad bottom base 54a and a narrow upper base-54b. A boxshaped water-cooled steel ring 540 is bolted to the bottom of the steel shell 54. A steel plate 54d is welded atop the steel shell 54 to form the narrow upper base 54b and to provide support for burners 29. The steel shell 54 is lined with a castable refractory lining 55 supported by a steel ring 55a. The bottom surface of the box-shaped water cooled steel ring 54c rests upon and forms a seal with the inverted Ushaped ring 42 atop the container 16 when the burner hood is in position to be fired. A plurality of downwardly directed burners 29 are positioned on the steel plate 54d. A plurality of flared openings 29a are provided in the refractory lining 55 to generally develop the shape of the flame from the burners 29 and direct the hot combustion gases downwardly toward the scrap charge in the container 16.

The combination of the conical shape of the burner hood 28, the flared openings 29a in the castable refractory and the geometrical configuration of the burner heads (shown in FIG. 3) in the burner hood 28 results in a uniform distribution of hot combustion gases in the burner hood 28. The above conditions provide a radiant dome effect which further enhances the uniform distribution of hot combustion gases in the burner hood. The hot combustion gases flow uniformly downwardly from the burner hood 28 into the upper portions of the scrap thereby uniformly heating the scrap. Of

course, it must be understood that as the hot combus-- tion gases pass downwardly through the scrap the uniformity of the flow can be altered. While there is nonuniform flow of hot combustion gases through the center and bottom portions of the scrap charge, the scrap is heated to a mean temperature, for example, l,500 F., and the bottom portions of the scrap are heated to a temperature, for example, about 750 F.

The shape of the burner hood also results in a substantially noiseless firing operation and allows the use of a flame which is relatively short and is contained in the burner hood area. Since the flame does not extend downwardly into the container, the flame does not impinge upon the upper surface areas of the scrap charge thereby minimizing oxidation of the scrap charge. Blowout of gases and flame between the burner hood and container is virtually eliminated because of the combined effect of the uniform downward flow of gases and the downwardly flared shape of the burner hood. The burners can fire gaseous carbonaceous fuel, for example, natural gas, and can use air to control the flame temperature in the hood to, for example, about 2,300 P. The volumetric heat release in the hood can be, for example, about 1,300 BTU/minute/cubic foot. The heat flux at the exit of the burner hood can be, for example, about 4,870 BTU/minute/square foot and a gas can have a velocity of about 8.5 feet per second.

FIG. 3 shows the arrangement of the downwardly directed burners 29 and the flared openings 29a in the refractory lining 55. The box-shaped water-cooled steel ring shown at 540 is provided with a water inlet pipe 54k and water inlet hose 54s, a water outlet pipe 54h and water outlet hose 54f, and a baffle plate 54g. The baffle plate 54g directs the water introduced through inlet pipe 542 to flow completely around the box-shaped water-cooled steel ring 540 to the outlet pipe 54). Thus. a continuous flow of cold water is assured in the boxshaped water-cooled steel ring 54c. The inclusion of the box-shaped water-cooled steel ring 54c prevented warpage of refractory support 55a in the refractorylined burner hood 28 and also prevented warpage of the steel support ring 42 atop the container 16. Unexpectedly, the seal formed between the box-shaped water-cooled steel ring 54c and the steel support ring 42 atop the container 16 was sufficiently tight to substantially prevent leakage of hot combustion gases, flame egress and also aided in making the operation substantially noiseless. While we have shown seven burners as a preferred geometric configuration, it must be understood that more or less burners can be used in other geometric configurations to achieve the results of the invention.

In operation, the container 16 is charged with a low density or high density scrap charge in a scrap yard. The density of the scrap charged into the container 16 is the ratio of the weight of the scrap to that volume of the container occupied by the scrap-We have classified scrap charges as low density scrap charges if the density of the charged scrap is between about 60 pounds per cubic foot to about 120 pounds per cubic foot. Low density scrap charges usually contain primarily light scrap with some heavy scrap added thereto. We have also classified scrap charges as high density'scrap charges if the density of the charged scrap is between about 120 pounds per cubic foot to about 200 pounds per cubic foot. High density scrap charges usually contain primarily heavy scrap but can contain light scrap.

The scrap is charged into the container 16 to obtain a uniform distribution across the container 16 and a minimum amount of channels are formed in the scrap charge. Naturally formed voids allow the downward passage of the hot combustion gases through the scrap charge. The container 16 which is carried on the selfpropelled transfer car is transported to the preheater station. The refractory-lined burner hood 28 with its plurality of downwardly directed burners 29 is placed atop the supporting ring 42 of the container 16. The slidable exhaust duct 38 is moved into place against the bottom of the transfer car floor 17a to form a continuous passage with the orifices 52 in the partible jaws 23, 23a of the clam shell. The annular passage in the floor 17a of the transfer car 17 and the exhaust system 34 beneath the floor 13 of the shop allows the spent hot combustion gases to be exhausted.

r The burners 29 are preferably of the air-natural gas type. The flame temperature is controlled by adjusting the ratio of the air and the gaseous carbonaceous fuel. The design of the burner hood is such that the short flame formed in the burner hood does not extend downwardly beyond the bottom base of the burner hood. The flame does not impinge upon the upper layers of the scrap charge in the container regardless of the fuel rate. This is of particular importance when low density scrap charges are preheated since light scrap in the upper layers of the charge can be easily oxidized and melted. It is of relatively little or no consequence when high density scrap charges are preheated because the upper layers of the scrap charge contain heavy example, l,500 F., in the scrap charge and controlling the rate of heat input for a time to soak the scrap charge. The above described heating cycle prevents rapid heating of the upper layers of the scrap charge thereby preventing oxidation and melting of the scrap charge. Then, too,'oil which contaminates light scrap is vaporized but is not ignited. Therefore, the problems of excessive oxidation and melting of the scrap charge caused by flame impingement on the scrap charge and melting of the scrap charge due to high temperatures caused by burning oil are minimized, if not completely eliminated.

High density scrap charges can be heated in a cycle which includes an initial rapid rate of heat input, for example, about 45 MM BTU per hour for atime to obtain a temperature of about 750 F. in the bottom of the container and regulating the rate of heat input to maintain the temperature of about 750 F. for a time to soak the scrap charge and obtain a mean temperature, for example, about l,500 F. (The designation MM represents million and has found acceptance among plant personnel as a way of designating heat input.)

. It must be recognized that low density scrap charges can be heatedat relatively higher firing rates than mentioned above if the upper layer of the scrap charge comprises heavy scrap because the heat input will be absorbed by the heavy scrap, thereby preventing melting, fusion and oxidation of the light scrap. Conversely, a high density scrap charge which has light scrap charged on the top layer is heated at a relatively lower rate of firing than mentioned above to prevent oxida- The temperature of the scrap in the upper layers of the scrap charge are as high as 2,300 F'.- to 2,400 F. at the end of the heating cycle. However, the temperature of the scrap must be below the melting and fusion temperatures thereof. Under the above conditions, the

in a specific example of the invention, a high density scrap charge of 130,400 poundsjof heavy scrap comprising large plates, small plates, bloom butts, 1 H bundles and molds and stools, having a volume of 720 cubic feet and a density of 180 pounds per cubic foot was fired for about 1 hour and 46 minutes, the initial firing rate being 45 MM BTU per hour until an exit temperature of 750 F. was reached in the bottom of the container. The 750 F. exit temperature was reached after 25-minutes of firing time. Decreasing firing rates as low as 25 MM BTU per hour were used to maintain the temperature at 750 F. for the remainder of the firing cycle. The firing was concluded, the firing hood raised and the scrap charged into an electric furnace. The total fuel was 52 MM BTU. No problems with melting, fusion or excessive oxidation of the scrap were encountered.

In another specific example of the invention, a low density scrap charge of 132,400 pounds comprising molds and stools, pitfines, No. 2 bundles, l H bloom butts, 2 H shredded automobile and fragmentized automobile scrap was charged into a container. The volume of scrap was 1,960 cubic feet and the charge density was 68 pounds per cubic foot. The container was placed on a transfer car and positioned in a preheater station. The firing hood was placed in position atop the container and the slidable duct placed in operation position. The scrap charge was fired at a rate of about 10 MM BTU per hour for an initial 14 minutes and between 25 MM BTU/hour to MM BTU/hour for an ddi o min te f a to firins i cqtfi m utes. The temperature of the spent hot combustion gases flowing out of the container through the clam shell was 750 F. Firing was stopped and the preheated scrap charged into an electric furnace. Preheating scrap charges prior to charging the scrap into an electric furnace effectively decreases the time required to melt and refine the scrap charge and the power used to a melt and refine the scrap charge when compared to the use of cold scrap charges. The time required to melt and refine three scrap charges preheated substantially the same as the second specific example was 11.7 percent less and the power 11.9 percent less than the time and power required to melt and refine cold scrap charges in the same electric furnace.

We claim:

1. In a method for preheating scrap charge in a container, said scrap charge having a density which is determined by the ratio of the weight of said scrap charge to the volume occupied by said scrap charge, said method comprising forming hot combustion gases uniformlydistributed in a refractory-lined burner hood positioned atop said container, passing said hot combustion gases from said refractory-lined burner hood uni- 4 tion and possible melting and fusion of the light scrap.

formly downwardly through said scrap charge to'heat said scrap charge at a first rate for a time to raise the temperature of said scrap charge to a first level and subsequently increasing the rate of heating said scrap charge for a time to raise the temperature of the scrap to a second level and regulating the rate of heating to thereby uniformly heat the scrap.

2. An improved method for preheating a scrap charge consisting of light scrap and heavyscrap in a container in a preheater station wherein a burner hood lined with refractory is placed atop said container and hot combustiongases are caused to flow uniformly from said burner hood downwardly through said scrap charge and are passedoutwardly through the bottom of the container to an exhaust system, said method comprising: i Y a. providing a burner hood with a plurality of downwardly directed burner heads adapted to burn a mixture of gaseous carbonaceous fuel and air to form said hot combustion gases in said burner hood, the shape of said burner'hood, and the geometric configuration of said burner heads causing (b) is at a rate between about 10 MM BTU per hour to about 45 MM BTU per hour.

5. The method of claim 4 in which the heating of step (b) is at a rate between about 10 MM BTU per hour to about 45 MM BTU per hour.

6. The method of claim 2 in which the scrap charge has a density of between about 60 pounds per cubic foot to about l20 pounds per cubic foot and the heating in step (b) includes heating the scrap charge at a first heating rate of between about 10 MM BTU per hour to about MM BTU per hour for between about minutes to about 30 minutes and a second heating rate of not more than 30 MM BTU per hour for a time to obtain a mean temperature of about l,500 F. in the scrap charge.

7. The method of claim 2 in which the scrap charge has a density of between about 120 pounds per cubic foot to about 200 pounds per cubic foot and the heating of step (b) includes a first heating rate of about 45 MM BTU per hour to obtain a temperature of 750 F. in the bottom of the container and a second heating rate sufficient to maintain said temperature in said bottom of said container.

8. In an improved method for preheating scrap in a container containing scrap having a density of between about 60 pounds per cubic foot to about 200 pounds per cubic foot, scrap being heated by hot combustion gases flowing downwardly through said scrap, the im-v per hour for a time to obtain a temperature of about 750 F. in the bottom of the container.

9. In an improved method for preheating scrap in a container containing scrap having a density of between about 60 pounds per cubic foot to about pounds per cubic foot, said scrap being heated by hot combustion gases flowing downwardly through said scrap, the improvement comprising heating the scrap at a rate of between about 10 MM BTU per hour to about 15 MM BTU per hour for a time to vaporize oil on said scrap without igniting same and increasing the rate of heating to not more than 30 MM BTU per hour for a time to obtain a mean temperature of about 750 F. in the bottom of the container.

10. In an improved method for preheating scrap in a container containing scrap having a density of between about l20 pounds per cubic foot to about 200 pounds per cubic foot, said scrap being heated by hot combustion gases flowing downwardly through said scrap, the improvement comprising heating said scrap at a rate of about 45 MM BTU per hour for a time to raise the temperature in the bottom of said container to about 750 F. and regulating the rate of heating to maintain said temperature in said bottom of container. 

1. In a method for preheating scrap charge in a container, said scrap charge having a density which is determined by the ratio of the weight of said scrap charge to the volume occupied by said scrap charge, said method comprising forming hot combustion gases uniformly distributed in a refractory-lined burner hood positioned atop said container, passing said hot combustion gases from said refractory-lined burner hood uniformly downwardly through said scrap charge to heat said scrap charge at a first rate for a time to raise the temperature of said scrap charge to a first level and subsequently increasing the rate Of heating said scrap charge for a time to raise the temperature of the scrap to a second level and regulating the rate of heating to thereby uniformly heat the scrap.
 2. An improved method for preheating a scrap charge consisting of light scrap and heavy scrap in a container in a preheater station wherein a burner hood lined with refractory is placed atop said container and hot combustion gases are caused to flow uniformly from said burner hood downwardly through said scrap charge and are passed outwardly through the bottom of the container to an exhaust system, said method comprising: a. providing a burner hood with a plurality of downwardly directed burner heads adapted to burn a mixture of gaseous carbonaceous fuel and air to form said hot combustion gases in said burner hood, the shape of said burner hood, and the geometric configuration of said burner heads causing a uniform distribution of heat in said hot combustion gases in said burner hood and a uniform distribution of heat and downward flow of said hot combustion gases, and b. heating said scrap charge in said container at a rate of heat input predicated on the density of said scrap charge.
 3. The method of claim 2 in which the density of the scrap charge is between about 60 pounds per cubic foot to about 200 pounds per cubic foot.
 4. The method of claim 2 in which the heating of step (b) is at a rate between about 10 MM BTU per hour to about 45 MM BTU per hour.
 5. The method of claim 4 in which the heating of step (b) is at a rate between about 10 MM BTU per hour to about 45 MM BTU per hour.
 6. The method of claim 2 in which the scrap charge has a density of between about 60 pounds per cubic foot to about 120 pounds per cubic foot and the heating in step (b) includes heating the scrap charge at a first heating rate of between about 10 MM BTU per hour to about 15 MM BTU per hour for between about 20 minutes to about 30 minutes and a second heating rate of not more than 30 MM BTU per hour for a time to obtain a mean temperature of about 1, 500* F. in the scrap charge.
 7. The method of claim 2 in which the scrap charge has a density of between about 120 pounds per cubic foot to about 200 pounds per cubic foot and the heating of step (b) includes a first heating rate of about 45 MM BTU per hour to obtain a temperature of 750* F. in the bottom of the container and a second heating rate sufficient to maintain said temperature in said bottom of said container.
 8. In an improved method for preheating scrap in a container containing scrap having a density of between about 60 pounds per cubic foot to about 200 pounds per cubic foot, scrap being heated by hot combustion gases flowing downwardly through said scrap, the improvements comprising heating said scrap at a rate between about 10 MM BTU per hour to about 45 MM BT per hour for a time to obtain a temperature of about 750* F. in the bottom of the container.
 9. In an improved method for preheating scrap in a container containing scrap having a density of between about 60 pounds per cubic foot to about 120 pounds per cubic foot, said scrap being heated by hot combustion gases flowing downwardly through said scrap, the improvement comprising heating the scrap at a rate of between about 10 MM BTU per hour to about 15 MM BTU per hour for a time to vaporize oil on said scrap without igniting same and increasing the rate of heating to not more than 30 MM BTU per hour for a time to obtain a mean temperature of about 750* F. in the bottom of the container.
 10. In an improved method for preheating scrap in a container containing scrap having a density of between about 120 pounds per cubic foot to about 200 pounds per cubic foot, said scrap being heated by hot combustion gases flowing downwardly through said scrap, The improvement comprising heating said scrap at a rate of about 45 MM BTU per hour for a time to raise the temperature in the bottom of said container to about 750* F. and regulating the rate of heating to maintain said temperature in said bottom of container. 